The present invention relates to an apparatus and a method for cold working a material and, more particularly, to an apparatus and a method for cold working a material by separately applying various forces to the material in a single application.
In aerospace structures, materials such as structural members including spars, stringers, ribs and an outer skin, e.g., aluminum sheeting or planar material, may be joined together by the use of fasteners. For example, a stringer having an engaging or faying surface may be juxtaposed with a corresponding faying surface of a planar material. Thereafter, holes or apertures may be drilled into the materials and fasteners, such as rivets, inserted into the apertures to form a fastened joint.
The fastened joints are potentially subject to failure from fatigue by the formation of fissures or cracks in the materials after substantial use. In airplanes, for example, cyclical pressurization and depressurization during each flight causes various stress cycles on the fuselage skin. These stress cycles likely contribute to a formation of cracks, which typically start at the apertures in the material where fasteners join the airplane skin to the skeleton or rib structure of the airplane. This is because, e.g., varying loads caused by pressurization changes that are carried by the skin must be routed around the periphery of the apertures.
During assembly of the skin of an airplane, such as along the fuselage and along the wing structure, it is well known to first cold work the apertures prior to assembly with a fastener in order to create a residual compressive stress about the periphery of the aperture. The residual compressive stress functions to counteract a loading that generally includes, in the case of fuselage pressurization changes, tensile strain variations. In this way, the useful life of the fuselage skin is greatly enhanced.
Current cold working methods require completion of a substantial number of steps. For example, a prior art method for cold working a material (which may include a joint) is diagrammatically shown at 10 in FIG. 1. In a first drill step 11, a drill 12 is used to create an aperture 14 in a material 16 to be cold worked. In a subsequent inspect step 18, an inspection device 20 may be employed to inspect the aperture 14 to determine if the diameter is within tolerance. Thereafter, in a cold work step 22, a mandrel 24 having a diameter that is about 3% larger than that of the aperture 14 is forced through the aperture 14 from one side of the material 16. Accordingly, the aperture 14 is thereby expanded which compresses the material surrounding the aperture 14 so as to create a residual stress field in the material 16 surrounding the aperture 14. In a ream step 28, the aperture 14 may be bored to a proper size for receiving a fastener (not shown) using a ream 30. In a counter sink step 32, the aperture 14 is counter sunk with a bit 34 to recess a head of a fastener (not shown). Thereafter, in a cleaning step 36, a solvent may be used by a mechanic 40 to remove lubricating oil from the cold work step, prior to another inspection step 42 using the inspection device 20.
Recently, it has been proposed that the material located, e.g., at a joint, be compressed or coined, prior to creating an aperture, so as to create a residual stress field which extends within a material to be fastened. One example of such a two step method is shown in FIG. 2a where in a coining step 43, a pair of indentors 44 and 46 are urged under a force in the direction of arrows 50, 52 into contact with a material 48.
As shown in greater detail in FIG. 2b, the indentors 44 and 46 each include a blunt end 54, 56 and a shoulder 58, 60 in fixed relationship. The blunt ends 54, 56 function to create a residual stress field, represented by arrows 62, which extends deep within the material 48 to a joint 64. The shoulders 58, 60 function to create a larger diametrical residual stress field, represented by arrows 66, but which extends generally closely to surfaces 68, 70 of the material 48. The use of shoulders 58, 60 is to overcome the tensile stress created at the part surface by the blunt ends 54, 56. The depth and configuration of the residual stress field is dependent on, e.g., the force applied to the indentors 44, 46 and a length between the blunt ends 54, 56 and the shoulders 58, 60. Generally, it is desired that the residual stress field created by the shoulders 58, 60 remain near the surface of the material while the stress field created by the blunt ends 54, 56 extend to the joint 64.
Thereafter and referring again to FIG. 2a, in a drill and counter sink step 72, an aperture 74 is created by a drill and a counter sink bit 76.
While suitable for its intended purpose, a problem arises in that the apparatus and the method illustrated in FIGS. 2a and 2b requires employing a different indentor with a different configuration depending on the desired depth and configurations of the residual stress fields. This is particularly cumbersome in the manufacture of airplanes as there are numerous joints to be fastened, most of which, vary in depth from the skin.
Accordingly, there is a need for an improved apparatus and method for cold working that does not require the changing of the indentors depending on depth of the joint.
According to one aspect of the present invention, a passive-adaptive indentor is provided for cold working a material that includes a material surface and a depth extending in a direction that is perpendicular to a direction of the material surface. The passive-adaptive indentor comprises a first member that includes a first working end that is configured to create a first residual stress field that extends to a predetermined depth in the material. The indentor also comprises a second member that includes a second working end that is configured to create a second residual stress field that extends to a generally fixed second depth of the material regardless of the depth to which the first residual stress field extends.
In another aspect of the invention, a passive-adaptive indentor is provided for cold working a material that includes a material surface and a depth extending in a direction that is perpendicular to a direction of the material surface. The passive-adaptive indentor comprises a first member that has a first working end that is configured to create a first residual stress field and which extends to a predetermined depth in the material. The first member is subjected to a first force whereby the first working end strikes the material surface at a force sufficient to create the first residual stress field. A second member is disposed in slidable relationship with the first member and comprises a second working end. The second member is configured to create a second residual stress field that extends to a generally fixed second depth of the material regardless of the depth at which the first residual stress field extends. The second member also is subjected to a second force whereby the second working end strikes the material surface at a force sufficient to create the second residual stress field.
In a further aspect of the invention, a passive-adaptive indentor is provided for cold working a material that includes an aerospace structure. The aerospace structure includes a planar material having a planar material faying surface, a support material having a support material faying surface and a joint located at a contact portion of the planar material faying surface and the support material faying surface. The planar material includes a planar material surface and a depth extending in a direction that is perpendicular to a direction of the planar material surface. The passive-adaptive indentor comprises a first member that is configured to create a first residual stress field in the material that extends to the joint. The first member may be subjected to a first force whereby the first working end strikes the planar material surface at a force sufficient to create the first residual stress field and the first member may comprise a shaft terminating in a first working end which comprises an end surface. The indentor may also comprise a second member that is subjected to a second force whereby the second working end strikes the planar material surface at a force sufficient to create a second residual stress field that extends to a depth that is substantially less than that of the first residual stress field. The second member may comprise a tubular structure that terminates in a second working end that comprises a shoulder surface. The tubular structure may be dimensioned and configured to be in a co-axial relationship with the shaft and to be movable along an axial direction of the shaft. The tubular structure may also comprise a collar and a radially extending portion. The indentor may further comprise a housing having a cavity defined by an inner surface and an opening and the housing is in fixed relationship with the shaft which extends within the cavity and through the opening. The tubular structure also extends through the opening. The indentor may still further comprise a compressible spring material which comprises an elastomer that is disposed between the inner surface of the cavity and a surface of the radially extending portion of the tubular structure.
In still a further aspect of the invention, a method of cold working a material using a passive-adaptive indentor to create plural stress fields in the material is provided. The indentor is capable of varying a depth at which a first stress field is to extend while a second stress field extends to a fixed depth in the material. The material includes a material surface, a depth extending in a direction that is perpendicular to a direction of the material surface and the method comprises the steps of: providing a material having multiple locations to be cold worked; identifying a first location and a particular depth of interest to which a first stress field is to extend at the first location; identifying a particular force to apply to a passive-adaptive indentor depending upon the particular depth of interest; applying the particular force to the first passive-adaptive indentor to simultaneously create a first residual stress field that extends to the particular depth of interest and a second residual stress field that extends to a generally fixed depth of the material regardless of the depth to which the first residual stress field extends; and moving the passive-adaptive indentor to another location.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.